Term insider trading Definition: The buying and selling of corporate stock or other financial instruments based on knowledge that is not widely available to the general public. Insider trading is most often undertaken by corporate executives or directors using information that they have acquired by working "inside" the company. Insider trading is illegal because it gives an unfair advantage to those on the inside. The president of a pharmaceutical company might be inclined to sell stock in the company using advanced information that the government is about to decline the patent application for a new drug.